In a secondary working process of a wire, the wire is subjected to a heat treatment before drawing. Usually, the heat treatment is continuously carried out on the wire in a strand form, from the viewpoint of job efficiency. To this end, the wires to be supplied to a heat treatment furnace are continuous ones which are connected by butt welding or the like. More specifically, as shown in FIG. 1, the terminal end part of a wire (a) and the initial end part of a wire (b) are connected by such a butt welding operation. A welding burr exists at the connected part. Although the welding burr differs variously according to the welding conditons, a wire having a diameter of 1.0 - 15 mm will give rise to a burr whose width, or axial extent, taken in the traveling direction of the wire, is about 3 - 10 mm and whose diameter is about 1.5 - 40 mm. Such welding burrs are cut off and discarded at the point of take-up of the wire after completion of the heat treatment, so that they are not mixed in a coil of the wire so taken up. Accordingly, an operator engaged in the take-up of the wire after it has been subjected to such heat treatment has been responsible for conducting the job of reliably cutting off and discarding the welding burr parts during the take-up of the wire, while continuously observing them. In order to facilitate the detection of the welding burr part, i.e., the connected position of the wire, the welding burr part has hitherto beeen made conspicuous or attractive by winding a fine iron wire or the like round the welding burr part. The use of the fine iron wire or the like is based on the fact that, since the stranded wire passes through the heat treatment furnace at a high temperature, conventional display means, such as coloration, cannot be utilized for the detection of the connected position. The fine iron wire, or the like, is suitable as one which keeps its original form even when heated to the extent of the heat treatment temperature. All the pertinent jobs, however, rely on manpower. In particular, the passage of the wire subjected to the heat treatment need be continually observed in order to detect the connected positions. This has required much labor. Moreover, the detection precision has tended to become lower, especially as the number of wires to be processed and the processing speed thereof is increased.
In order to eliminate the disadvantages thereby encountered, there has recently been proposed a method in which, as disclosed in Japanese Patent Application Public-disclosure No. 2654/1975, the connected wire part is detected by a phototube switch, or the like, having a contactless mechanism. However, this method is not yet completely satisfactory, primarily because the size of the welding burr changes largely in dependence, not only on the wire diameter, but also on the welding conditions, etc. Thus, in the phototube switch having such a contactless mechanism, the adjustment of the light projecting face is a difficult job, particularly in such cases where the welding burr lies at the connected part at a change from a wire of larger diameter to a wire of smaller diameter, whereby the detection precision for the connected part generally lowers. Besides, the phototube switch must be arranged for each individual wire to be detected, so that the method is disadvantageous in cost.
U.S. Pat. No. 3,812,424 has proposed an expedient in which the cross sectional area of a wire is gauged by measuring an electrostatic capacity. Herein, a sensing electrode is formed in a manner to surround one wire. In order to detect defects of a plurality of wires at the same time, therefore, a number of electrodes are required according to the number of the wires. It is accordingly difficult to detect the welded joints of the plurality of wires at a stroke. Further means to detect defective parts by measuring electrostatic capacities are taught in U.S. Pat. Nos. 2,565,500; 3,052,826; 3,519,922; 3,684,089; 3,764,899; 3,827,296 and 3,879,660 and DT-OS's No. 1,918,715; 2,137,545 and 2,025,644. None of them, however, can be said to be an effective and satisfactory technique for detecting a welded joint, an entangled part and/or a disconnected part of a wire in the continuous heat treatment of the wire, and especially where a plurality of wires are being considered at once. Accordingly, the development of a technique for successfully meeting this purpose is desired.